feat: implement 8 features across full stack

- Leaderboard: rank-based percentile tiers (champion/gold/silver/bronze),
    deterministic tie-breaking (xp DESC, user_id ASC)
  - Mentor chat: md: breakpoints, safe-area-inset-bottom padding
  - Learning progress: server-side progressPct, curriculum expansion safety
  - Physical orders: hard-disabled with 410 Gone
  - Certificate approval: pending/issued flow, admin issue/revoke endpoints,
    certificate_requires_approval setting (migration 0031)
  - Draft/publish: is_published on lessons and chapters, admin publish UI
    (migration 0030)
  - Video panel: static thumbnails (no preload), client-side pagination
  - Rich editor: TipTap + tiptap-markdown for lesson descriptions,
    marked + DOMPurify for SafeMarkdown rendering

backend/internal/db/migrations/0030_lesson_draft_status.sql

@@ -0,0 +1,9 @@
+-- Add draft/published state to chapters and lessons.
+-- Existing content is published immediately so nothing goes dark on deploy.
+-- New lessons created via the admin API default to is_published = false (draft).
+
+ALTER TABLE lessons  ADD COLUMN IF NOT EXISTS is_published BOOLEAN NOT NULL DEFAULT false;
+ALTER TABLE chapters ADD COLUMN IF NOT EXISTS is_published BOOLEAN NOT NULL DEFAULT false;
+
+UPDATE lessons  SET is_published = true;
+UPDATE chapters SET is_published = true;